In recent years, it has been actively studied that semiconductor devices, including a blue LED, a white LED, a blue-violet semiconductor laser, etc., are fabricated by using a nitride of a group 13 element, such as gallium nitride, and that those semiconductor devices are applied to various types of electronic devices. Conventional gallium nitride-based semiconductor devices are produced mainly by a vapor phase method. Specifically, a gallium-nitride-based semiconductor device is produced by hetero-epitaxially growing a thin film of gallium nitride on a sapphire body or a silicon carbide substrate by a metal organic vapor phase epitaxy (MOVPE) and the like. In this case, the substrate and the thin film of gallium nitride differ greatly from each other in coefficient of thermal expansion and lattice constant, thus generating dislocations (as one kind of lattice defects in crystals) in gallium nitride at high density. Thus, it is difficult to obtain a high-quality gallium nitride with a low dislocation density by the vapor phase method.
A flux method is one of liquid phase methods. In the case of gallium nitride, metallic sodium is used as the flux, which can relieve conditions required for crystal growth of gallium nitride, namely, a temperature of approximately 800° C. and a pressure of several MPa. Specifically, nitrogen gas is dissolved in a mixed melt composed of metallic sodium and metallic gallium, and the gallium nitride is in an oversaturated state to grow as crystal. Such a liquid phase method is less likely to generate dislocations as compared to the vapor phase method, and thus can produce the high-quality gallium nitride with a low dislocation density.
The present applicant has filed the following applications as methods for producing a GaN template using a Na flux method: Patent document 1 (Japanese Unexamined Patent Application Publication No. 2010-168236A); Patent document 2 (WO 2013/022122A); Patent document 3 (WO 2013/021804A); and Patent document 4 (WO 2013/022123A).
Further, Patent document 5 (Japanese Unexamined Patent Application Publication No. 2005-136167A) discloses that, in order to correct warpage of a GaN self-supporting body, the front side and back side of the GaN self-supporting body are polished in turn.